The present invention relates to measuring the thickness of a coating material layer of a running strip such as a strip of galvanized steel.
In order to protect steel sheets against corrosion, they are coated with a material such as zinc and galvanized sheets are thereby obtained. In order to produce this zinc coating, the strips are run in an annealing oven and then through a bell in order to have them penetrate into a liquid zinc bath so that they emerge therefrom coated with a zinc layer which is liquid and which is dried by blowing a gas such as nitrogen. After drying, the strips are optionally subject to a heat treatment so as to cause a reaction between the zinc layer and the steel substrate, and the sheets are then cooled and finally they are introduced into a surface rolling installation such as a «skin-pass mill» before being wound. In order to obtain sheets which have a nice regular surface so that, after having been painted, they have an excellent surface aspect, it is required that the zinc or zinc alloy coating layer be as smooth as possible. Further, in order to be able to guarantee good protection against corrosion, it is required that the coating layer be of a sufficient thickness in any point of the strip. In order to obtain this result on industrial installations reliably, it is both necessary to be able to control the process and check the result and therefore measure the thickness of the zinc layer. In order to conduct this thickness measurement, methods with X fluorescence are generally used, which give the possibility of making an absolute measurement of the thickness of the zinc layer but which have the drawback of being relatively slow. Because of its slowness, this process does not give the possibility of producing a dense mapping of the surface of the strip and therefore does not give the possibility of carrying out strict quality control. For the same reason, it only allows detection of process operational irregularities, the changes in variations of which are small. Consequently, by measuring the thickness of the coating by X fluorescence, it is not possible to achieve sufficiently strict control of the coating line in order to obtain a guarantee on the quality of the products, which is required for meeting the needs of the market.
In order to allow regulation of the thickness of a conducting coating layer on a ferrous substrate, the use of a measurement with an eddy current sensor with a high frequency of 500 kHz was proposed. But this method is not sufficiently accurate and is not intended for producing a mapping of the coating thickness of a strip.
The object of the present invention is to find a remedy to this drawback by proposing a means allowing measurement of the thickness of the coating layer on a running strip, which is sufficiently fast and accurate in order to be able to produce a dense mapping of the thickness of the coating layers on the strip and for allowing tight control of the coating line so as to be able to regulate relatively fast operational irregularities of the process.